According to protocol on greenhouse gases  scope 3 emissions include all indirect emissions that occur in your company's value chain. 

Unlike the other two emissions, scope 1 (direct emissions from own or controlled sources) and scope 2 (indirect emissions from purchased electricity, heat or steam), scope 3 emissions capture a wider range of impacts. These emissions are often more difficult to measure and control because they are very diverse and diffuse in nature. 

Scope 3 emissions fall into three different categories:

1. Upstream emissions : These emissions occur in the supply chain and include activities such as extraction of raw materials, production and transportation of goods and services.
2. Subsequent emissions : This category includes emissions related to the use, disposal and processing of the company's end-of-life products.
3. Value chain emissions : Value chain emissions, which span the entire life cycle of a product or service, include both upstream and downstream impacts.

(Jennifer L, more at carboncredits.com)